Electromagnetic device

ABSTRACT

An electromagnetic device comprises a magnetic field generating electrical circuit including a winding ( 1 ) having at least one electrical conductor ( 2 ). The winding comprises a solid insulation ( 4 ) surrounded by outer and inner layers ( 3, 5 ) serving for equalization of potential and having semiconducting properties. Said at least one electrical conductor ( 2 ) is arranged interiorly of the inner semiconducting layer ( 3 ). The invention also relates to methods for electric field control and production of a magnetic circuit as well as use of a cable for obtaining a winding.

TECHNICAL FIELD

This invention is related to an electromagnetic device for electricpower purposes, comprising a magnetic field generating electric circuitincluding at least one electric conductor having an insulation system.This electromagnetic device may be used in any electrotechnicalconnection. The power range may be from VA up to the 1000-MVA range.High voltage applications are primarily intended, up to the highesttransmission voltages used today.

According to a first aspect of the invention a rotating electric machineis contemplated. Such electric machines comprise synchronous machineswhich are mainly used as generators for connection to distribution andtransmission networks, commonly referred to below as power networks. Thesynchronous machines are also used as motors and for phase compensationand voltage control, in that case as mechanically idling machines. Thetechnical field also comprises double-fed machines, asynchronousconverter cascades, external pole machines, synchronous flux machinesand asynchronous machines.

According to another aspect of the invention, said electromagneticdevice is formed by a power transformer or reactor. For all transmissionand distribution of electric energy, transformers are used and theirtask is to allow exchange of electric energy between two or moreelectric systems and for this, electromagnetic induction is utilized ina well-known manner. The transformers primarily intended with thepresent invention belong to the so-called power transformers with arated power of from a few hundred kVA up to more than 1000 MVA with arated voltage of from 3-4 kV and up to very high transmission voltages,400 kV to 800 kV or higher.

Although the following description of the prior art with respect to thesecond aspect mainly refers to power transformers, the present inventionis also applicable to reactors, which, as is well-known, may be designedas single-phase and three-phase reactors. As regards insulation andcooling there are, in principle, the same embodiments as fortransformers. Thus, air-insulated and oil-insulated, self-cooled,pressure-oil cooled, etc., reactors are available. Although reactorshave one winding (per phase) and may be designed both with and without amagnetic core, the description of the background art is to a largeextent relevant also to reactors.

The magnetic field inducing electric circuit may in some embodiments beair-wound but comprises as a rule a magnetic core of laminated, normalor oriented, sheet or other, for example amorphous or powder-based,material, or any other action for the purpose of allowing an alternatingflux, and a winding. The circuit often comprises some kind of coolingsystem etc. In the case of a rotating electric machine, the winding maybe disposed in the stator or the rotor of the machine, or in both.

The invention also comprises a method for electric field control in anelectromagnetic device and a method for production of a magneticcircuit.

PRIOR ART

In order to be able to explain and describe the invention, the prior artwill be discussed hereinafter both in respect of a rotating electricmachine and a power transformer.

Rotating Electric Machine

Such a rotating electric machine will be exemplified based upon asynchronous machine. The first part of the description substantiallyrelates to the magnetic circuit of such a machine and how it is composedaccording to classic technique. Since the magnetic circuit referred toin most cases is disposed in the stator, the magnetic circuit below willnormally be described as a stator with a laminated core, the winding ofwhich will be referred to as a stator winding, and the slots in thelaminated core for the winding will be referred to as stator slots orsimply slots.

Most synchronous machines have a field winding in the rotor, where themain flux is generated by direct current, and an ac winding in thestator. The synchronous machines are normally of three-phase design.Sometimes, the synchronous machines are designed with salient poles. Thelatter have an ac winding in the rotor.

The stator body for large synchronous machines are often made of sheetsteel with a welded construction. The laminated core is normally, madefrom varnished 0.35 or 0.5 mm electric sheet. For larger machines, thesheet is punched into segments which are attached to the stator body bymeans of wedges/dovetails. The laminated core is retained by pressurefingers and pressure plates.

For cooling of the windings of the synchronous machine, three differentcooling systems are available.

In case of air cooling, both the stator winding and the rotor windingare cooled by cooling air flowing through. The cooling air channels areto be found both in the stator laminations and in the rotor. For radialventilation and cooling by means of air, the sheet iron core at leastfor medium-sized and large machines is divided into stacks with radialand axial ventilation ducts disposed in the core. The cooling air mayconsist of ambient air but at high power a closed cooling system withheat exchangers is substantially used. Hydrogen cooling is used inturbogenerators and in large synchronous compensators. The coolingmethod functions in the same way as in air cooling with heat exchangers,but instead of air as coolant there is used hydrogen gas. The hydrogengas has better cooling capacity than air, but difficulties arise atseals and in monitoring leakage. For turbogenerators in the higher powerrange it is known to apply water cooling of both the stator winding andthe rotor winding. The cooling channels are in the form of tubes whichare placed inside conductors in the stator winding. One problem withlarge machines is that the cooling tends to become non-uniform and that,therefore, temperature differences arise across the machine.

The stator winding is disposed in slots in the sheet iron core, theslots normally having a cross section as that of a rectangle or atrapezoid. Each winding phase comprises a number of series-connectedcoil groups and each coil group comprises a number of series-connectedcoils. The different parts of the coil are designated coil side for thatpart which is placed in the stator and coil end for that part which isdisposed outside the stator. A coil comprises one or more conductorsbrought together in height and/or width. Between each conductor there isa thin insulation, for example epoxy/glass fibre.

The coil is insulated against the slot with a coil insulation, that is,an insulation intended to withstand the rated voltage of the machine toground. As insulating material, various plastic, varnish and glass fibrematerials may be used. Usually, so-called mica tape is used, which is amixture of mica and hard plastic, especially produced to provideresistance to partial discharges, which can rapidly break down theinsulation. The insulation is applied to the coil by winding the micatape around the coil in several layers. The insulation is impregnated,and then the coil side is painted with a coal-based paint to improve thecontact with the surrounding stator which is connected to groundpotential.

The conductor area of the windings is determined by the currentintensity in question and by the cooling method used. The conductor andthe coil are usually formed with a rectangular shape to maximize theamount of conductor material in the slot. A typical coil is formed ofso-called Roebel bars, in which certain of the bars may be made hollowfor a coolant. A Roebel bar comprises a plurality of rectangular,parallel-connected copper conductors, which are transposed 360 degreesalong the slot. Ringland bars with transpositions of 540 degrees andother transpositions also occur. The transposition is made to avoid theoccurrence of circulating currents which are generated in a crosssection of the conductor material, as viewed from the magnetic field.

For mechanical and electrical reasons, a machine cannot be made in justany size. The machine power is determined substantially by threefactors:

The conductor area of the windings. At normal operating temperature,copper, for example, has a maximum value of 3-3.5 A/mm².

The maximum flux density (magnetic flux) in the stator and rotormaterial.

The maximum electric field strength in the insulating material, theso-called dielectric strength.

Polyphase ac windings are designed either as single-layer or two-layerwindings. In the case of single-layer windings, there is only one coilside per slot, and in the case of two-layer windings there are two coilsides per slot. Two-layer windings are usually designed as diamondwindings, whereas the single-layer windings which are relevant in thisconnection may be designed as a diamond winding or as a concentricwinding. In the case of a diamond winding, only one coil span (orpossibly two coil spans) occurs, whereas flat windings are designed asconcentric windings, that is, with a greatly varying coil width. By coilwidth is meant the distance in circular measure between two coil sidesbelonging to the same coil, either in relation to the relevant polepitch or in the number of intermediate slot pitches. Usually, differentvariants of chording are used, for example fractional pitch, to give thewinding the desired properties. The type of winding substantiallydescribes how the coils in the slots, that is, the coil sides, areconnected together outside the stator, that is, at the coil ends.

Outside the stacked sheets of the stator, the coil is not provided witha painted semiconducting ground-potential layer. The coil end isnormally provided with an E-field control in the form of so-calledcorona protection varnish intended to convert a radial field into anaxial field, which means that the insulation on the coil ends occurs ata high potential relative to ground. This sometimes gives rise to coronain the coil-end region, which may be destructive. The so-calledfield-controlling points at the coil ends entail problems for a rotatingelectric machine.

Normally, all large machines are designed with a two-layer winding andequally large coils. Each coil is placed with one side in one of thelayers and the other side in the other layer. This means that all thecoils cross each other in the coil end. If more than two layers areused, these crossings render the winding work difficult and deterioratethe coil end.

It is generally known that the connection of a synchronousmachine/generator to a power network must be made via a Δ/Y-connectedso-called step-up transformer, since the voltage of the power networknormally lies at a higher level than the voltage of the rotatingelectric machine. Together with the synchronous machine, thistransformer thus constitutes integrated parts of a plant. Thetransformer constitutes an extra cost and also entails the disadvantagethat the total efficiency of the system is lowered. If it were possibleto manufacture machines for considerably higher voltages, the step-uptransformer could thus be omitted.

During the last few decades, there have been increasing requirements forrotating electric machines for higher voltages than what has previouslybeen possible to design. The maximum voltage level which, according tothe state of the art, has been possible to achieve for synchronousmachines with a good yield in the coil production is around 25-30 kV.

Certain attempts to a new approach as regards the design of synchronousmachines are described, inter alia, in an article entitled“Water-and-oil-cooled Turbogenerator TVM-300” in J. Elektrotechnika, No.1, 1970, pp. 6-8, in U.S. Pat. No. 4,429,244 “Stator of Generator” andin Russian patent document CCCP Patent 955369.

The water- and oil-cooled synchronous machine described in J.Elektrotechnika is intended for voltages up to 20 kV. The articledescribes a new insulation system consisting of oil/paper insulation,which makes it possible to immerse the stator completely in oil. The oilcan then be used as a coolant while at the same time using it asinsulation. To prevent oil in the stator from leaking out towards therotor, a dielectric oil-separating ring is provided at the internalsurface of the core. The stator winding is made from conductors with anoval hollow shape provided with oil and paper insulation. The coil sideswith their insulation are secured to the slots made with rectangularcross section by means of wedges. As coolant oil is used both in thehollow conductors and in holes in the stator walls. Such coolingsystems, however, entail a large number of connections of both oil andelectricity at the coil ends. The thick insulation also entails anincreased radius of curvature of the conductors, which in turn resultsin an increased size of the winding overhang.

The above-mentioned U.S. patent relates to the stator part of asynchronous machine which comprises a magnetic core of laminated sheetwith trapezoidal slots for the stator winding. The slots are taperedsince the need of insulation of the stator winding is smaller towardsthe interior of the rotor where that part of the winding which islocated nearest the neutral point is disposed. In addition, the statorpart comprises a dielectric oil-separating cylinder nearest the innersurface of the core. This part may increase the magnetizationrequirement relative to a machine without this ring. The stator windingis made of oil-immersed cables with the same diameter for each coillayer. The layers are separated from each other by means of spacers inthe slots and secured by wedges. What is special for the winding is thatit comprises two so-called half-windings connected in series. One of thetwo half-windings is disposed, centred, inside an insulating sleeve. Theconductors of the stator winding are cooled by surrounding oil.Disadvantages with such a large quantity of oil in the system are therisk of leakage and the considerable amount of cleaning work which mayresult from a fault condition. Those parts of the insulating sleevewhich are located outside the slots have a cylindrical part and aconical termination reinforced with current-carrying layers, the duty ofwhich is to control the electric field strength in the region where thecable enters the end winding.

From CCCP 955369 it is clear, in another attempt to raise the ratedvoltage of the synchronous machine, that the oil-cooled stator windingcomprises a conventional high-voltage cable with the same dimension forall the layers. The cable is placed in stator slots formed as circular,radially disposed openings corresponding to the cross-section area ofthe cable and the necessary space for fixing and for coolant. Thedifferent radially disposed layers of the winding are surrounded by andfixed in insulating tubes. Insulating spacers fix the tubes in thestator slot. Because of the oil cooling, an internal dielectric ring isalso needed here for sealing the oil coolant against the internal airgap. The design also exhibits a very narrow radial waist between thedifferent stator slots, which means a large slot leakage flux whichsignificantly influences the magnetization requirement of the machine.

A report from Electric Power Research Institute, EPRI, EL-3391, from1984 describes a review of machine concepts for achieving a highervoltage of a rotating electric machine for the purpose of being able toconnect a machine to a power network without an intermediatetransformer. Such a solution is judged by the investigation to providegood efficiency gains and great economic advantages. The main reasonthat it was considered possible in 1984 to start developing generatorsfor direct connection to power networks was that at that time asuperconducting rotor had been produced. The large magnetizationcapacity of the superconducting field makes it possible to use an airgap winding with a sufficient thickness to withstand the electricalstresses. By combining the most promising concept, according to theproject, of designing a magnetic circuit with a winding, a so-calledmonolith cylinder armature, a concept where the winding comprises twocylinders of conductors concentrically enclosed in three cylindricalinsulating casings and the whole structure is fixed to an iron corewithout teeth, it was judged that a rotating electric machine for highvoltage could be directly connected to a power network. The solutionmeant that the main insulation had to be made sufficiently thick to copewith network-to-network and network-to-ground potentials. The insulationsystem which, after a review of all the technique known at the time, wasjudged to be necessary to manage an increase to a higher voltage wasthat which is normally used for power transformers and which consists ofdielectric-fluid-impregnated cellulose pressboard. Obvious disadvantageswith the proposed solution are that, in addition to requiring asuperconducting rotor, it requires a very thick insulation whichincreases the size of the machine. The coil ends must be insulated andcooled with oil or freons to control the large electric fields in theends. The whole machine must be hermetically enclosed to prevent theliquid dielectric from absorbing moisture from the atmosphere.

When manufacturing rotating electric machines according to the state ofthe art, the winding is manufactured with conductors and insulationsystems in several steps, whereby the winding must be preformed prior tomounting on the magnetic circuit. Impregnation for preparing theinsulation system is performed after mounting of the winding on themagnetic circuit.

Power Transformer/Reactor

To be able to place a power transformer/reactor according to theinvention in its proper context and hence be able to describe the newapproach which the invention means as well as the advantages afforded bythe invention in relation to the prior art, a relatively completedescription of a power transformer as it is currently designed willfirst be given below as well as of the limitations and problems whichexist when it comes to calculation, design, insulation, grounding,manufacture, use, testing, transport, etc., of these transformers.

From a purely general point of view, the primary task of a powertransformer is to allow exchange of electric energy between two or moreelectrical systems of, normally, different voltages with the samefrequency.

A conventional power transformer comprises a transformer core, in thefollowing referred to as a core, often of laminated oriented sheet,usually of silicon iron. The core comprises a number of core limbs,connected by yokes which together form one or more core windows.Transformers with such a core are often referred to as coretransformers. Around the core limbs there are a number of windings whichare normally referred to as primary, secondary and control windings. Asfar as power transformers are concerned, these windings are practicallyalways concentrically arranged and distributed along the length of thecore limbs. The core transformer normally has circular coils as well asa tapering core limb section in order to fill up the coils as closely aspossible.

Also other types of core designs are known, for example those which areincluded in so-called shell-type transformers. These are often designedwith rectangular coils and a rectangular core limb section.

Conventional power transformers, in the lower part of theabove-mentioned power range, are sometimes designed with air cooling tocarry away the unavoidable inherent losses. For protection againstcontact, and possibly for reducing the external magnetic field of thetransformer, it is then often provided with an outer casing providedwith ventilating openings.

Most of the conventional power transformers, however, are oil-cooled.One of the reasons therefor is that the oil has the additional veryimportant function as insulating medium. An oil-cooled and oil-insulatedpower transformer is therefore surrounded by an external tank on which,as will be clear from the description below, very high demands areplaced. Normally, means for water-cooling of the coil are provided.

The following part of the description will for the most part refer tooil-filled power transformers.

The windings of the transformer are formed from one or severalseries-connected coils built up of a number of series-connected turns.In addition, the coils are provided with a special device to allowswitching between the terminals of the coils. Such a device may bedesigned for changeover with the aid of screw joints or more often withthe aid of a special changeover switch which is operable in the vicinityof the tank. In the event that changeover can take place for atransformer under voltage, the changeover switch is referred to as anon-load tap changer whereas otherwise it is referred to as ade-energized tap changer.

Regarding oil-cooled and oil-insulated power transformers in the upperpower range, the breaking elements of the on-load tap changers areplaced in special oil-filled containers with direct connection to thetransformer tank. The breaking elements are operated purely mechanicallyvia a motor-driven rotating shaft and are arranged so as to obtain afast movement during the switching when the contact is open and a slowermovement when the contact is to be closed. The on-load tap changers assuch, however, are placed in the actual transformer tank. During theoperation, arcing and sparking arise. This leads to degradation of theoil in the containers. To obtain less arcs and hence also less formationof soot and less wear on the contacts, the on-load tap changers arenormally connected to the high-voltage side of the transformer. This isdue to the fact that the currents which need to be broken and connected,respectively, are smaller on the high-voltage side than if the on-loadtap changers were to be connected to the low-voltage side. Failurestatistics of conventional oil-filled power transformers show that it isoften the on-load tap changers which give rise to faults.

In the lower power range of oil-cooled and oil-insulated powertransformers, both the on-load tap changers and their breaking elementsare placed inside the tank. This means that the above-mentioned problemswith degradation of the oil because of arcs during operation, etc.,effect the whole oil system.

From the point of view of applied or induced voltage, it can broadly besaid that a voltage which is stationary across a winding is distributedequally onto each turn of the winding, that is, the turn voltage isequal on all the turns.

From the point of view of electric potential, however, the situation iscompletely different. One end of a winding is normally connected toground. This means, however, that the electric potential of each turnincreases linearly from practically zero in the turn which is nearestthe ground potential up to a potential in the turns which are at theother end of the winding which correspond to the applied voltage.

This potential distribution determines the composition of the insulationsystem since it is necessary to have sufficient insulation both betweenadjacent turns of the winding and between each turn and ground.

The turns in an individual coil are normally brought together into ageometrical coherent unit, physically delimited from the other coils.The distance between the coils is also determined by the dielectricstress which may be allowed to occur between the coils. This thus meansthat a certain given insulation distance is also required between thecoils. According to the above, sufficient insulation distances are alsorequired to the other electrically conducting objects which are withinthe electric field from the electric potential locally occurring in thecoils.

It is thus clear from the above description that for the individualcoils, the voltage difference internally between physically adjacentconductor elements is relatively low whereas the voltage differenceexternally in relation to other metal objects—the other coils beingincluded—may be relatively high. The voltage difference is determined bythe voltage induced by magnetic induction as well as by the capacitivelydistributed voltages which may arise from a connected externalelectrical system on the external connections of the transformer. Thevoltage types which may enter externally comprise, in addition tooperating voltage, lightning overvoltages and switching overvoltages.

In the current leads of the coils, additional losses arise as a resultof the magnetic leakage field around the conductor. To keep these lossesas low as possible, especially for power transformers in the upper powerrange, the conductors are normally divided into a number of conductorelements, often referred to as strands, which are parallel-connectedduring operation. These strands must be transposed according to such apattern that the induced voltage in each strand becomes as identical aspossible and so that the difference in induced voltage between each pairof strands becomes as small as possible for internally circulatingcurrent components to be kept down at a reasonable level from the losspoint of view.

When designing transformers according to the prior art, the general aimis to have as large a quantity of conductor material as possible withina given area limited by the so-called transformer window, generallydescribed as having as high a fill factor as possible. The availablespace shall comprise, in addition to the conductor material, also theinsulating material associated with the coils, partly internally betweenthe coils and partly to other metallic components including the magneticcore.

The insulation system, partly within a coil/winding and partly betweencoils/windings and other metal parts, is normally designed as a solidcellulose- or varnish-based insulation nearest the individual conductorelement, and outside of this as solid cellulose and liquid, possiblyalso gaseous, insulation. Windings with insulation and possible bracingparts in this way represent large volumes which will be subjected tohigh electric field strengths which arise in and around the activeelectromagnetic parts of the transformer. To be able to predetermine thedielectric stresses which arise and achieve a dimensioning with aminimum risk of breakdown, good knowledge of the properties ofinsulating materials is required. It is also important to achieve such asurrounding environment that it does not change or reduce the insulatingproperties.

The currently predominant insulation system for high-voltage powertransformers comprises cellulose material as the solid insulation andtransformer oil as the liquid insulation. The transformer oil is basedon so-called mineral oil.

The transformer oil has a dual function since, in addition to theinsulating function, it actively contributes to cooling of the core, thewinding, etc., by removal of the loss heat of the transformer. Oilcooling requires an oil pump, an external cooling element, an expansioncoupling, etc.

The electrical connection between the external connections of thetransformer and the immediately connected coils/windings is referred toas a bushing aiming at a conductive connection through the tank which,in the case of oil-filled power transformers, surrounds the actualtransformer. The bushing is often a separate component fixed to the tankand is designed to withstand the insulation requirements being made,both on the outside and the inside of the tank, while at the same timeit should withstand the current loads occurring and the ensuing currentforces. It should be pointed out that the same requirements for theinsulation system as described above regarding the windings also applyto the necessary internal connections between the coils, betweenbushings and coils, different types of changeover switches and thebushings as such.

All the metallic components inside a power transformer are normallyconnected to a given ground potential with the exception of thecurrent-carrying conductors. In this way, the risk of an unwanted, anddifficult-to-control, potential increase as a result of capacitvevoltage distribution between current leads at high potential and groundis avoided. Such an unwanted potential increase may give rise to partialdischarges, so-called corona. Corona may be revealed during the normalacceptance tests, which partially occurs, compared with rated data,increased voltage and frequency. Corona may give rise to damage duringoperation.

The individual coils in a transformer must have such a mechanicaldimensioning that they may withstand any stresses occurring as aconsequence of currents arising and the resultant current forces duringa short-circuit process. Normally, the coils are designed such that theforces arising are absorbed within each individual coil, which in turnmay mean that the coil cannot be dimensioned optimally for its normalfunction during normal operation.

Within a narrow voltage and power range of oil-filled powertransformers, the windings are designed as so-called sheet windings.This means that the individual conductors mentioned above are replacedby thin sheets. Sheet-wound power transformers are manufactured forvoltages of up to 20-30 kV and powers of up to 20-30 MW.

The insulation system of power transformers within the upper power rangerequires, in addition to a relatively complicated design, also specialmanufacturing measures to utilize the properties of the insulationsystem in the best way. For a good insulation to be obtained, theinsulation system shall have a low moisture content, the solid part ofthe insulation shall be well impregnated with the surrounding oil andthe risk of remaining “gas” pockets in the solid part must be minimal.To ensure this, a special drying and impregnating process is carried outon a complete core with windings before it is lowered into a tank. Afterthis drying and impregnating process, the transformer is lowered intothe tank which is then sealed. Before filling of oil, the tank with theimmersed transformer must be emptied of all air. This is done inconnection with a special vacuum treatment. When this has been carriedout, filling of oil takes place.

To be able to obtain the promised service life, etc., pumping out toalmost absolute vacuum is required in connection with the vacuumtreatment. This thus presupposes that the tank which surrounds thetransformer is designed for full vacuum, which entails a considerableconsumption of material and manufacturing time.

If electric discharges occur in an oil-filled power transformer, or if alocal considerable increase of the temperature in any part of thetransformer occurs, the oil is disintegrated and gaseous products aredissolved in the oil. The transformers are therefore normally providedwith monitoring devices for detection of gas dissolved in the oil.

For weight reasons large power transformers are transported without oil.In-situ installation of the transformer at a customer requires, in turn,renewed vacuum treatment. In addition, this is a process which,furthermore, has to be repeated each time the tank is opened for someaction or inspection.

It is obvious that these processes are very time-consuming andcost-demanding and constitute a considerable part of the total time formanufacture and repair while at the same time requiring access toextensive resources.

The insulating material in conventional power transformers constitutes alarge part of the total volume of the transformer. For a powertransformer in the upper power range, oil quantities in the order ofmagnitude of hundreds of cubic meters of transformer oil may occur. Theoil which exhibits a certain similarity to diesel oil is thinly fluidand exhibits a relatively low flash point. It is thus obvious that oiltogether with the cellulose constitutes a non-negligible fire hazard inthe case of unintentional heating, for example at an internal flashoverand a resultant oil spillage.

It is also obvious that, especially in oil-filled power transformers,there is a very large transport problem. Such a power transformer in theupper power range may have a total weight of up to 1 000 tons. It isrealized that the external design of the transformer must sometimes beadapted to the current transport profile, that is, for any passage ofbridges, tunnels, etc.

Here follows a short summary of the prior art with respect to oil-filledpower transformers and which may be described as limitation and problemareas therefor:

An oil-filled conventional power transformer

comprises an outer tank which is to house a transformer comprising atransformer core with coils, oil for insulation and cooling, mechanicalbracing devices of various kinds, etc. Very large mechanical demands areplaced on the tank, since, without oil but with a transformer, it shallbe capable of being vacuum-treated to practically full vacuum. The tankrequires very extensive manufacturing and testing processes and thelarge external dimensions of the tank also normally entail considerabletransport problems;

normally comprises a so-called pressure-oil cooling. This cooling methodrequires the provision of an oil pump, an external cooling element, anexpansion vessel and an expansion coupling, etc.;

comprises an electrical connection between the external connections ofthe transformer and the immediately connected coils/windings in the formof a bushing fixed to the tank. The bushing is designed to withstand anyinsulation requirements made, both regarding the outside and the insideof the tank;

comprises coils/windings whose conductors are divided into a number ofconductor elements, strands, which have to be transposed in such a waythat the voltage induced in each strand becomes as identical as possibleand such that the difference in induced voltage between each pair ofstrands becomes as small as possible;

comprises an insulation system, partly within a coil/winding and partlybetween coils/windings and other metal parts which is designed as asolid cellulose- or varnish-based insulation nearest the individualconductor element and, outside of this, solid cellulose and a liquid,possibly also gaseous, insulation. In addition, it is extremelyimportant that the insulation system exhibits a very low moisturecontent;

comprises as an integrated part an on-load tap changer, surrounded byoil and normally connected to the high-voltage winding of thetransformer for voltage control;

comprises oil which may entail a non-negligible fire hazard inconnection with internal partial discharges, so-called corona, sparkingin on-load tap changers and other fault conditions;

comprises normally a monitoring device for monitoring gas dissolved inthe oil, which occurs in case of electrical discharges therein or incase of local increases of the temperature;

comprises oil which, in the event of damage or accident, may result inoil spillage leading to extensive environmental damage.

SUMMARY OF THE INVENTION

The object of the present invention is primarily to provide anelectromagnetic device, in which at least one or some of thedisadvantages discussed hereinabove and impairing the prior art havebeen eliminated. Besides, the invention secondarily aims at devising amethod for electric field control in an electromagnetic device forelectric power purposes and a method for producing a magnetic circuitfor a rotating electric machine.

The primary object is achieved by means of a device of the kind definedin the following claims, and then first of all in the characterizingpart of any of claims 1-5.

In a wide sense, it is established that the design according to theinvention reduces the occurring losses such that the device,accordingly, may operate with a higher efficiency as a consequence ofthe fact that the invention makes it possible to substantially enclosethe electric field occurring due to said electric conductor in theinsulation system. The reduction of losses results, in turn, in a lowertemperature in the device, which reduces the need for cooling and allowspossibly occurring cooling devices to be designed in a more simple waythan without the invention.

The conductor/insulation system according to the invention may berealised as a flexible cable, which means substantial advantages withrespect to production and mounting as compared to the prefabricated,rigid windings which have been conventional up to now. The insulationsystem used according to the invention results in abscence of gaseousand liquid insulation materials.

As to the aspect of the invention as a rotating electric machine it isthus possible to operate the machine with such a high voltage that theΔ/Y-connected step-up transformer mentioned above can be omitted. Thatis, the machine can be operated with a considerably higher voltage thanmachines according to the state of the art to be able to perform directconnection to power networks. This means considerably lower investmentcosts for systems with a rotating electric machine and the totalefficiency of the system can be increased. The invention eliminates theneed for particular field control measures at certain areas of thewinding, such field control measures having been necessary according tothe prior art. A further advantage is that the invention makes it moresimple to obtain under- and overmagnetization for the purpose ofreducing reactive effects as a result of voltage and current being outof phase with each other.

As to the aspect of the invention as a power transformer/reactor, theinvention, first of all, eliminates the need for oil filling of thepower transformers and the problems and disadvantages associatedthereto.

The design of the winding so that it comprises, along at least a part ofits length, an insulation formed by a solid insulating material,inwardly of this insulation an inner layer and outwardly of theinsulation an outer layer with these layers made of a semi conductingmaterial makes it possible to enclose the electric field in the entiredevice within the winding. The term “solid insulating material” usedherein means that the winding is to lack liquid or gaseous insulation,for instance in the form of oil. Instead the insulation is intended tobe formed by a polymeric material. Also the inner and outer layers areformed by a polymeric material, though a semiconducting such.

The inner layer and the solid insulation are rigidly connected to eachother over substantially the entire interface. Also the outer layer andthe solid insulation are rigidly connected to each other oversubstantially the entire interface therebetween. The inner layeroperates equalizing with respect to potential and, accordingly,equalizing with respect to the electrical field outwardly of the innerlayer as a consequence of the semiconducting properties thereof. Theouter layer is also intended to be made of a semiconducting material andit has at least an electrical conductivity being higher than that of theinsulation so as to cause the outer layer, by connection to earth orotherwise a relatively low potential, to function equalizing with regardto potential and to substantially enclose the electrical field resultingdue to said electrical conductor inwardly of the outer layer. On theother hand, the outer layer should have a resistivity which issufficient to minimize electrical losses in said outer layer.

The rigid interconnection between the insulating material and the innerand outer semiconducting layers should be uniform over substantially theentire interface such that no cavities, pores or similar occur. With thehigh voltage levels contemplated according to the invention, theelectrical and thermal loads which may arise will impose extreme demandson the insulation material. It is known that so-called partialdischarges, PD, generally constitute a serious problem for theinsulating material in high-voltage installations. If cavities, pores orthe like arise at an insulating layer, internal corona discharges mayarise at high electric voltages, whereby the insulating material isgradually degraded and the result could be electric breakdown throughthe insulation. This may lead to serious breakdown of theelectromagnetic device. Thus, the insulation should be homogenous.

The inner layer inwardly of the insulation should have an electricalconductivity which is lower than that of the electrical conductor butsufficient for the inner layer to function equalizing with regard topotential and, accordingly, equalizing with respect to the electricalfield externally of the inner layer. This in combination with the rigidinterconnection of the inner layer and the electrical insulation oversubstantially the entire interface, i.e. the abscence of cavities etc,means a substantially uniform electrical field externally of the innerlayer and a minimum of risk for PD.

It is preferred that the inner layer and the solid electrical insulationare formed by materials having substantially equal thermal coefficientsof expansion. The same is preferred as far as the outer layer and thesolid insulation is concerned. This means that the inner and outerlayers and the solid electrical insulation will form an insulationsystem which on temperature changes expands and contracts uniformly as amonolithic part without those temperature changes giving rise to anydestruction or disintegration in the interfaces. Thus, intimacy in thecontact surface between the inner and outer layers and the solidinsulation is ensured and conditions are created to maintain thisintimacy during prolonged operation periods.

The electrical load on the insulation system decreases as a consequenceof the fact that the inner and the outer layers of semiconductingmaterial around the insulation will tend to form substantiallyequipotential surfaces and in this way the electrical field in theinsulation properly will be distributed relatively uniformly over thethickness of the insulation.

It is known, per se, in connection with transmission cables forhigh-voltage and for transmission of electric energy, to designconductors with an insulation of a solid insulation material with innerand outer layers of semiconducting material. In transmission of electricenergy, it has since long been realised that the insulation should befree from defects. However, in high voltage cables for transmission, theelectric potential does not change along the length of the cable but thepotential is basically at the same level. However, also in high voltagecables for transmission purposes, instantaneous potential differencesmay occur due to transient occurrencies, such as lightning. According tothe present invention a flexible cable according to the enclosed claimsis used as a winding in the electromagnetic device.

An additional improvement may be achieved by constructing the electricconductor in the winding from smaller, so-called strands, at least someof which are insulated from each other. By making these strands to havea relatively small cross section, preferably approximately circular, themagnetic field across the strands will exhibit a constant geometry inrelation to the field and the occurrence of eddy currents are minimized.

According to the invention, the winding/windings is/are thus preferablymade in the form of a cable comprising at least one conductor and thepreviously described insulation system, the inner layer of which extendsabout the strands of the conductor. Outside of this inner semiconductinglayer is the main insulation of the cable in the form of a solidinsulation material.

The outer semiconducting layer shall according to the invention exhibitsuch electrical properties that a potential equalization along theconductor is ensured. The outer layer may, however, not exhibit suchconductivity properties that an induced current will flow along thesurface, which could cause losses which in turn may create an unwantedthermal load. For the inner and outer layers the resistance statements(at 20° C.) defined in the enclosed claims 8 and 9 are valid. Withrespect to the inner semiconducting layer, it must have a sufficientelectrical conductivity to ensure potential equalization for theelectrical field but at the same time this layer must have such aresistivity that the enclosing of the electric field is ensured. It isimportant that the inner layer equalizes irregularities in the surfaceof the conductor and forms an equipotential surface with a high surfacefinish at the interface with the solid insulation. The inner layer maybe formed with a varying thickness but to ensure an even surface withrespect to the conductor and the solid insulation, the thickness issuitably between 0.5 and 1 mm.

Such a flexible winding cable which is used according to the inventionin the electromagnetic device thereof is an improvement of a XLPE(cross-linked poly ethylene) cable or a cable with EP(ethylene-propylene) rubber insulation or other rubber, for examplesilicone. The improvement comprises, inter alia, a new design both asregards the strands of the conductors and in that the cable, at least insome embodiments, has no outer casing for mechanical protection of thecable. However, it is possible according to the invention to arrange aconducting metal shield and an outer mantel externally of the outersemiconducting layer. The metal shield will then have the character ofan outer mechanical and electrical protection, for instance tolightning. It is preferred that the inner semiconducting layer will lieon the potential of the electrical conductor. For this purpose at leastone of the strands of the electrical conductor will be uninsulated andarranged so that a good electrical contact is obtained to the innersemiconducting layer. Alternatively, different strands may bealternatingly brought into electrical contact with the innersemiconducting layer.

Manufacturing transformer or reactor windings of a cable according tothe above entails drastic differences as regards the electric fielddistribution between conventional power transformers/reactors and apower transformer/reactor according to the invention. The decisiveadvantage with a cable-formed winding according to the invention is thatthe electric field is enclosed in the winding and that there is thus noelectric field outside the outer semiconducting layer. The electricfield achieved by the current-carrying conductor occurs only in thesolid main insulation. Both from the design point of view and themanufacturing point of view this means considerable advantages:

the windings of the transformer may be formed without having to considerany electric field distribution and the transposition of strands,mentioned under the background art, is omitted;

the core design of the transformer may be formed without having toconsider any electric field distribution;

no oil is needed for electrical insulation of the winding, that is, themedium surrounding the winding may be air;

no special connections are required for electrical connection betweenthe outer connections of the transformer and the immediately connectedcoils/windings, since the electrical connection, contrary toconventional plants, is integrated with the winding;

the manufacturing and testing technology which is needed for a powertransformer according to the invention is considerably simpler than fora conventional power transformer/reactor since the impregnation, dryingand vacuum treatments described under the description of the backgroundart are not needed. This provides considerably shorter production times;

by using the technique according to the invention for insulation,considerable possibilities are provided for developing the magnetic partof the transformer, which was given according to the prior art.

In application of the invention as a rotating electric machine asubstantially reduced thermal load on the stator is obtained. Temporaryoverloads of the machine will, thus, be less critical and it will bepossible to drive the machine at overload for a longer period of timewithout running the risk of damage arising. This means considerableadvantages for owners of power generating plants who are forced today,in case of operational disturbances, to rapidly switch to otherequipment in order to ensure the delivery requirements laid down by law.

With a rotating electric machine according to the invention, themaintenance costs can be significantly reduced because transformers andcircuit breakers do not have to be included in the system for connectingthe machine to the power network.

Above it has already been described that the outer semiconducting layerof the winding cable is intended to be connected to ground potential.The purpose is that the layer should be kept substantially on groundpotential along the entire length of the winding cable. It is possibleto divide the outer semiconducting layer by cutting the same into anumber of parts distributed along the length of the winding cable, eachindividual layer part being connectable directly to ground potential. Inthis way a better uniformity along the length of the winding cable isachieved.

Above it has been mentioned that the solid insulation and the inner andouter layers may be achieved by, for instance, extrusion. Othertechniques are, however, also well possible, for instance formation ofthese inner and outer layers and the insulation respectively by means ofspraying of the material in question onto the conductor/winding.

It is preferred that the winding cable is designed with a circular crosssection. However, also other cross sections may be used in cases whereit is desired to achieve a better packing density.

To build up a voltage in the rotating electric machine, the cable isdisposed in several consecutive turns in slots in the magnetic core. Thewinding can be designed as a multilayer concentric cable winding toreduce the number of coil-end crossings. The cable may be made withtapered insulation to utilize the magnetic core in a better way, inwhich case the shape of the slots may be adapted to the taperedinsulation of the winding.

A significant advantage with a rotating electric machine according tothe invention is that the E field is near zero in the coil-end regionoutside the outer semiconductor and that with the outer casing at groundpotential, the electric field need not be controlled. This means that nofield concentrations can be obtained, neither within sheets, in coil-endregions or in the transition therebetween.

The present invention is also related to a method for electric fieldcontrol in an electromagnetic device for electric power purposes.

The invention also relates to a method for manufacturing a magneticcircuit, a flexible cable, which is threaded into openings in slots in amagnetic core of the rotating electrical machine being used as awinding. Since the cable is flexible, it can be bent and this permits acable length to be disposed in several turns in a coil. The coil endswill then consist of bending zones in the cables. The cable may also bejoined in such a way that its properties remain constant over the cablelength. This method entails considerable simplifications compared withthe state of the art. The so-called Roebel bars are not flexible butmust be preformed into the desired shape. Impregnation of the coils isalso an exceedingly complicated and expensive technique whenmanufacturing rotating electric machines today.

To sum up, thus, a rotating electric machine according to the inventionmeans a considerable number of important advantages in relation tocorresponding prior art machines. First of all, it can be connecteddirectly to a power network at all types of high voltage. By highvoltage are meant here voltages exceeding 10 kV and up to the voltagelevels which occur for power networks. Another important advantage isthat a chosen potential, for example ground potential, has beenconsistently conducted along the whole winding, which means that thecoil-end region can be made compact and that bracing means at thecoil-end region can be applied at practically ground potential or anyother chosen potential. Still another important advantage is thatoil-based insulation and cooling systems disappear also in rotatingelectric machines as already has been pointed out above with regard topower transformers/reactors. This means that no sealing problems mayarise and that the dielectric ring previously mentioned is not needed.One advantage is also that all forced cooling can be made at groundpotential.

BRIEF DESCRIPTION OF THE DRAWINGS

With reference to the enclosed drawings, a more specific description ofembodiment examples of the invention will follow hereinafter.

In the drawings:

FIG. 1 is a partly cut view showing the parts included in the currentmodified standard cable;

FIG. 2 is an axial end view of a sector/pole pitch of a magnetic circuitaccording to the invention;

FIG. 3 is a view showing the electric field distribution around awinding of a conventional power transformer/reactor;

FIG. 4 is a perspective view showing an embodiment of a powertransformer according to the invention;

FIG. 5 is a cross section illustrating a cable structure modifiedrelative to FIG. 1 and having several electrical conductors; and

FIG. 6 is a cross section of a further cable structure comprisingseveral electric conductors but in another arrangement than that in FIG.5.

DESCRIPTION OF PREFERRED EMBODIMENTS

Rotating Electric Machine According to FIGS. 1 and 2

An important condition for being able to manufacture a magnetic circuitin accordance with the description of the invention, is to use for thewinding a conductor cable with a solid electrical insulation with aninner semiconducting layer or casing between the insulation and one ormore electrical conductors located inwardly therof and with an outersemiconducting layer or casing located outwardly of the insulation. Suchcables are available as standard cables for other power engineeringfields of use, namely power transmission. To be able to describe anembodiment, initially a short description of a standard cable will bemade. The inner current-carrying conductor comprises a number ofnon-insulated strands. Around the strands there is a semiconductinginner layer. Around this semiconducting inner layer, there is aninsulating layer of solid insulation. The solid insulation is formed bya polymeric material with low electrical losses and a high breakthroughstrength. As concrete examples polyethylene (PE) and then particularlycross-linked polyethylene (XLPE) and ethylene-propylene (EP) may bementioned. Around the outer semiconducting layer a metal shield and anouter insulation casing may be provided. The semiconducting layersconsist of a polymeric material, for example ethylene-copolymer, with anelectrically conducting constituent, e.g. conductive soot or carbonblack. Such a cable will be referred to hereunder as a power cable.

A preferred embodiment of a cable intended for a winding in a rotatingelectrical machine appears from FIG. 1. The cable 1 is described in thefigure as comprising a current-carrying conductor 2 which comprisestransposed both non-insulated strands 2A and insulated strands 2B.Electromechanically transposed, extruded insulated strands are alsopossible. These strands may be stranded/transposed in a plurality oflayers. Around the conductor there is an inner semiconducting layer 3which, in turn, is surrounded by a homogenous layer of a solidinsulation material. The insulation 4 is entirely without insulationmaterial of liquid or gaseous type. This layer 4 is surrounded by anouter semiconducting layer 5. The cable used as a winding in thepreferred embodiment may be provided with metal shield and externalsheath but must not be so. To avoid induced currents and lossesassociated therewith in the outer semiconducting layer 5, this is cutoff, preferably in the coil end, that is, in the transitions from thesheet stack to the end windings. The cut-off is carried such that theouter semiconducting layer 5 will be divided into several partsdistributed along the cable and being electrically entirely or partlyseparated from each other. Each cut-off part is then connected toground, whereby the outer semiconducting layer 5 will be maintained at,or near, ground potential in the whole cable length. This means that,around the solid insulated winding at the coil ends, the contactablesurfaces, and the surfaces which are dirty after some time of use, onlyhave negligible potentials to ground, and they also cause negligibleelectric fields.

To optimize a rotating electric machine, the design of the magneticcircuit as regards the slots and the teeth, respectively, are ofdecisive importance. As mentioned above, the slots should connect asclosely as possible to the casing of the coil sides. It is alsodesirable that the teeth at each radial level are as wide as possible.This is important to minimize the losses, the magnetization requirement,etc., of the machine.

With access to a conductor for the winding such as for example, thecable described above, there are great possibilities of being able tooptimize the magnetic core from several points of view. In thefollowing, a magnetic circuit in the stator of the rotating electricmachine is referred to. FIG. 2 shows an embodiment of an axial end viewof a sector/pole pitch 6 of a machine according to the invention. Therotor with the rotor pole is designated 7. In conventional manner, thestator is composed of a laminated core of electric sheets successivelycomposed of sector-shaped sheets. From a back portion 8 of the core,located at the radially outermost end, a number of teeth 9 extendradially inwards towards the rotor. Between the teeth there are acorresponding number of slots 10. The use of cables 11 according to theabove among other things permits the depth of the slots for high-voltagemachines to be made larger than what is possible according to the stateof the art. The slots have a cross section tapering towards the rotorsince the need of cable insulation becomes lower for each winding layertowards the air gap. As is clear from the figure, the slot substantiallyconsists of a circular cross section 12 around each layer of the windingwith narrower waist portions 13 between the layers. With somejustification, such a slot cross section may be referred to as a “cyclechain slot”. In the embodiment shown in FIG. 2, cables with threedifferent dimensions of the cable insulation are used, arranged in threecorrespondingly dimensioned sections 14, 15 and 16, that is, in practicea modified cycle chain slot will be obtained. The figure also shows thatthe stator tooth can be shaped with a practically constant radial widthalong the depth of the whole slot.

In an alternative embodiment, the cable which is used as a winding maybe a conventional power cable as the one described above. The groundingof the outer semiconducting shield then takes place by stripping themetal shield and the sheath of the cable at suitable locations.

The scope of the invention accommodates a large number of alternativeembodiments, depending on the available cable dimensions as far asinsulation and the outer semiconductor layer etc. are concerned. Alsoembodiments with so-called cycle chain slots can be modified in excessof what has been described here.

As mentioned above, the magnetic circuit may be located in the statorand/or the rotor of the rotating electric machine. However, the designof the magnetic circuit will largely correspond to the above descriptionindependently of whether the magnetic circuit is located in the statorand/or the rotor.

As winding, a winding is preferably used which may be described as amultilayer, concentric cable winding. Such a winding means that thenumber of crossings at the coil ends has been minimized by placing allthe coils within the same group radially outside one another. This alsopermits a simpler method for the manufacture and the threading of thestator winding in the different slots. Since the cable used according tothe invention is relatively easily flexible, the winding may be obtainedby a comparatively simple threading operation, in which the flexiblecable is threaded into the openings 12 present in the slots 10.

Power Transformer/reactor (FIGS. 3 and 4)

FIG. 3 shows a simplified and fundamental view of the electric fielddistribution around a winding of a conventional powertransformer/reactor, where 17 is a winding and 18 a core and 19illustrates equipotential lines, that is, lines where the electric fieldhas the same magnitude. The lower part of the winding is assumed to beat ground potential.

The potential distribution determines the composition of the insulationsystem since it is necessary to have sufficient insulation both betweenadjacent turns of the winding and between each turn and ground. Thefigure thus shows that the upper part of the winding is subjected to thehighest insulation loads. The design and location of a winding relativeto the core are in this way determined substantially by the electricfield distribution in the core window.

The cable which can be used in the windings contained in the dry powertransformers/reactors according to the invention have been describedwith assistance of FIG. 1. The cable may, as stated before, be providedwith other, additional outer layers for special purposes, for instanceto prevent excessive electrical strains on other areas of thetransformer/reactor. From the point of view of geometrical dimension,the cables in question will have a conductor area which is between 2 and3000 mm² and an outer cable diameter which is between 20 and 250 mm.

The windings of a power transformer/reactor manufactured from the cabledescribed under the summary of the invention may be used both forsingle-phase, three-phase and polyphase transformers/reactorsindependently of how the core is shaped. One embodiment is shown in FIG.4 which shows a three-phase laminated core transformer. The corecomprises, in conventional manner, three core limbs 20, 21 and 22 andthe retaining yokes 23 and 24. In the embodiment shown, both the corelimbs and the yokes have a tapering cross section.

Concentrically around the core limbs, the windings formed with the cableare disposed. As is clear, the embodiment shown in FIG. 4 has threeconcentric winding turns 25, 26 and 27. The innermost winding turn 25may represent the primary winding and the other two winding turns 26 and27 may represent secondary windings. In order not to overload the figurewith too many details, the connections of the windings are not shown.Otherwise the figure shows that, in the embodiment shown, spacing bars28 and 29 with several different functions are disposed at certainpoints around the windings. The spacing bars may be formed of insulatingmaterial intended to provide a certain space between the concentricwinding turns for cooling, bracing, etc. They may also be formed ofelectrically conducting material in order to form part of the groundingsystem of the windings.

Alternative Cable Designs

In the cable variant illustrated in FIG. 5, the same referencecharacters as before are used, only with the addition of the letter acharacteristic for the embodiment. In this embodiment the cablecomprises several electric conductors 2 a, which are mutually separatedby means of insulation 4 a. Expressed in other words, the insulation 4 aserves both for insulation between individual adjacent electricalconductors 2 a and between the same and the surrounding. The differentelectrical conductors 2 a may be disposed in different manners, whichmay provide for varying cross-sectional shapes of the cable in itsentirity. In the embodiment according to FIG. 5 it is illustrated thatthe conductors 2 a are disposed on a straight line, which involves arelatively flat cross-sectional shape of the cable. From this it can beconcluded that the cross-sectional shape of the cable may vary withinwide limits.

In FIG. 5 there is supposed to exist, between adjacent electricalconductors, a voltage smaller than phase voltage. More specifically, theelectrical conductors 2 a in FIG. 5 are supposed to be formed bydifferent revolutions in the winding, which means that the voltagebetween these adjacent conductors is comparatively low.

As before, there is a semiconducting outer layer 5 a exteriorly of theinsulation 4 a obtained by a solid electrical insulation material. Aninner layer 3 a of a semiconducting material is arranged about each ofsaid electrical conductors 2 a, i.e. each of these conductors has asurrounding inner semiconducting layer 3 a of its own. This layer 3 awill, accordingly, serve potential equalizing as far as the individualelectrical conductor is concerned.

The variant in FIG. 6 uses the same reference characters as before onlywith addition of the letter b specific for the embodiment. Also in thiscase there are several, more specifically three, electrical conductors 2b. Phase voltage is supposed to be present between these conductors,i.e. a substantially higher voltage than the one occurring betweenconductors 2 a in the embodiment according to FIG. 5. In FIG. 6 there isan inner semiconducting layer 3 b inwardly of which the electricalconductors 2 b are arranged. Each of the electrical conductors 2 b is,however, enclosed by a further layer 30 of its own, with propertiescorresponding to the properties discussed hereinabove with regard to theinner layer 3 b. Between each further layer 30 and the layer 3 barranged thereabout, there is insulation material. Accordingly, thelayer 3 b will occur as a potential equalizing layer outside the furtherlayers 30 of semiconducting material belonging to the electricalconductors, said further layers 30 being connected to the respectiveelectrical conductor 2 b to be placed on the same potential as theconductor.

Possible Modifications

It is evident that the invention is not only limited to the embodimentsdiscussed above. Thus, the man skilled within this art will realise thata number of detailed modifications are possible when the basic conceptof the invention has been presented without deviating from this conceptas it is defined in the enclosed claims. As an example, it is pointedout that the invention is not only restricted to the specific materialselections exemplified above. Functionally equal materials may,accordingly, be used instead. As far as the manufacturing of theinsulation system according to the invention is concerned, it is pointedout that also other techniques than extrusion and spraying are possibleas long as intimacy between the various layers is achieved. Furthermore,it is pointed out that additional equipotential layers could bearranged. For example, one or more equipotential layers ofsemiconducting material could be placed in the insulation between thoselayers designated as “inner” and “outer” hereinabove.

What is claimed is:
 1. A high voltage electromagnetic device comprisinga winding, wherein said winding comprises a flexible cable including atleast one current-carrying conductor including a plurality of insulatedconductive elements and at least one uninsulated conductive element anda magnetically permeable, electric field confining insulating coversurrounding the conductor in contact with the at least one uninsulatedelement, said cable forming at least one uninterrupted turn in thecorresponding winding of said device.
 2. The device of claim 1, whereinthe cover comprises an insulating layer surrounding the conductor and anouter layer surrounding the insulating layer, said outer layer having aconductivity for establishing an equipotential surface around theconductor.
 3. The device of claim 1, wherein the cover comprises aplurality of layers joined together to form a monolithic structureincluding an insulating layer and wherein said joined together pluralityof layers are substantially void fre.
 4. The device of claim 1, whereinthe cover comprises a plurality of layers joined together to form amonolithic structure wherein the joined together layers havesubstantially the same temperature coefficient of expansion.
 5. Thedevice of claim 1, wherein the cover comprises a plurality of layersjoined together to form a monolithic structure such that the device isoperable at 100% overload for two hours.
 6. The device of claim 1,wherein the cover is operable to render the cable free of sensible endwinding loss.
 7. The device of claim 1, wherein the cover is operable torender the cable free of partial discharge and field control.
 8. Anelectromagnetic device comprising a magnetic field generating electriccircuit including at least one electric conductor for producing whenenergized an electric field and having an insulation system comprising amagnetically permeable electric field confining insulating coveringsurrounding the conductor including a solid insulation material and atleast one layer having an electric conductivity higher than theinsulation to equalize potential and to enclose the electric field,inwardly of the at least one layer, wherein the at least one layercomprises an inner layer surrounding the conductor, and the inner layerhas an electrical conductivity lower than the conductivity of theconductor for equalizing the electrical field exteriorly of the innerlayer, and wherein said conductor comprises a plurality of insulatedconductive elements and at least one uninsulated conductive element inelectrical contact with the inner layer.
 9. The device according toclaim 8, wherein said at least one conductor forms at least oneinduction turn.
 10. The device according to claim 8, wherein at leastone of the inner layer and the outer layer has a resistivity in therange of at least one of about 10⁻⁶ cm and bout 10 k cm, about 10⁻³ andabout 1000 cm, and about 1 and about 500 cm.
 11. The device according toclaim 8, wherein at least one of the inner layer and the outer layer hasa resistance in a range of about 50μ and about
 5. 12. The deviceaccording to claim 8, wherein the solid insulation at least one of theinner layer and the outer layer are formed of polymeric materials. 13.The device according to claim 8, wherein the inner layer and at leastone of the outer layer and the solid insulation are rigidly connected toeach other an along interface therebetween.
 14. The device according toclaim 8, wherein the inner layer and at least one of the outer layer andthe solid insulation are formed by materials having substantially equalthermal coefficients of expansion.
 15. The device according to claim 8,wherein the solid insulation comprises an extruded layer.
 16. The deviceaccording to claim 15, wherein at least one of the inner layer and theouter layer comprises an extruded layer simultaneously formed with theextruded layer of the solid insulation.
 17. The device according toclaim 8, wherein the conductor and the insulation system comprises awinding in the form of a flexible cable.
 18. The device according toclaim 8, wherein the conductor has an area in a range of about 2 andabout 3000 mm² and the cable has an external diameter in a range ofabout 20 and about 250 mm.
 19. The device according to claim 8, whereinat least one of the inner layer and the outer layer comprises apolymeric material including an electrically conducting component. 20.The device according to claim 8, wherein the insulation system isoperable at high voltage in excess of at least one 10 kV, 36 kV and 72.5kV.
 21. The device according to claim 8, wherein the outer layer isdivided into a plurality of parts, separately connected to at least oneof ground or otherwise a relatively low potential.
 22. The deviceaccording to claim 8, comprising at least two galvanically separatedconcentrically wound windings.
 23. The device according to claim 8, saiddevice comprising a rotating electric machine.
 24. The machine accordingto claim 23, further comprising at least one of a stator and rotorforming a magnetic field generating electrical circuit for said machine.25. The machine according to claim 23, wherein the magnetic fieldgenerating circuit comprises at least one magnetic core having slots forreceiving the winding.
 26. The machine according to claim 25 wherein themachine has a coil-end-region and the electrical field outside theinsulation system is about zero in the slots and in the coil-end-regionwhen an outer layer of the insulation system is grounded.
 27. Themachine according to claim 23, wherein the slots have the shape ofcylindrical openings separated by a narrower waist portionstherebetween.
 28. The machine according to claim 27, wherein the slotshave a cross section which decreases inwardly of the magnetic core. 29.The machine according to claim 28, wherein the slots have a crosssection which decreases.
 30. The machine according to claim 23,comprising at least one of a generator, a motor, a synchronouscompensator, a transformer and a reactor.
 31. The method according toclaim 23, wherein the magnetic field generating circuit is arranged inat least one of a stator and a rotor of a rotating electric machineincluding a magnetic core having slots for the winding, said slots beingformed with openings, the winding comprising a flexible cable threadedinto the openings.
 32. An electromagnetic device comprising a magneticfield generating electric circuit including at least one electricconductor for producing an electric field when energized and aninsulation system surrounding the conductor, wherein the insulationsystem comprises an electric insulation formed by a solid insulatingmaterial and inwardly thereof an inner layer, said at least one electricconductor being located inwardly of the inner layer, the inner layerhaving an electric conductivity lower than the electric conductor forequalizing the electric field exteriorly of the inner layer, whereinsaid conductor comprises a plurality of insulated conductive elementsand at least one uninsulated conductive element in electrical contactwith the inner layer.
 33. The device according to claim 32, wherein theinsulation system further comprises an outer layer having an electricconductivity which is higher than the insulation to equalize and enclosethe electric field inwardly of the outer layer.
 34. The device accordingto claim 32, wherein the insulation system includes an outermost layerhaving an electric conductivity higher than the insulation.
 35. Thedevice according to claim 34, wherein the inner and outermost layers areformed of semiconducting materials.
 36. An electromagnetic devicecomprising at least one electric conductor having an insulation systemcomprising an insulated covering surrounding the conductor, including aninner layer of semiconducting material, an outer layer of semiconductingmaterial and a solid insulation material between the inner and outerlayers, the inner and outer layers and the solid insulation havingsubstantially the same thermal properties, wherein said at least oneelectric conductor comprises a plurality of insulated conductiveelements and at least one uninsulated conductive element in electricalcontact with the inner layer.
 37. A method for electric field control inan electromagnetic device comprising a magnetic field generating circuithaving at least one winding for producing, when energized, an electricfield, including least one electric conductor and an electric insulationexternally thereof, wherein the insulation is formed by a solidinsulation material and an outer layer externally of the insulationmaterial, said outer layer being connected to a relatively low potentialand having an electrical conductivity higher than the conductivity ofthe insulation and lower than the conductivity of the electricalconductor for equalizing potential and confining the electrical fieldwithin the outer layer, wherein said at least one electric conductorcomprises a plurality of insulated conductive elements and at least oneuninsulated conductive element in electrical contact with the innerlayer.